Recently, there have been reported increasing levels of research activities on holographic digital data storage systems triggered by the development of semiconductor lasers, e.g., charge coupled devices (CCDs) and the like. Since the holographic digital data storage system normally features a large storage capacity and a high data transfer rate, it has already been applied to, e.g., fingerprint recognition systems for storing and reproducing fingerprints and the scope of its applications keeps expanding.
The holographic digital data storage system allows a signal beam transmitted from an object to interfere with a reference beam and writes thus generated interference patterns on a storage medium made of, e.g., a photoreflective crystal or polymer which reacts to each pattern differently depending on the amplitude and phase of the interference pattern.
FIG. 1 depicts a block diagram of a holographic digital data storage system, which includes a light source 10, a beam splitter 20, two reflection mirrors 30 and 40, a spatial light modulator (SLM) 50, a storage medium 60, a CCD 70, a data arranging block 110 and a data extracting block 120.
The light source 10 generates a laser beam. The beam splitter 20 splits the laser beam into a reference beam and a signal beam and transfers the separated reference and signal beams along two different optical paths, wherein the reference beam and the signal beam correspond to a transmitted beam and a reflected beam, respectively.
The reference beam is reflected at the reflection mirror 30 so that the reflected reference beam is transferred to the storage medium 60. The signal beam, on the other hand, is reflected at the reflection mirror 40 so that the reflected signal beam is transferred to the SLM 50.
In the meantime, binary input data to be stored is arranged on a page basis at the data arranging block 110. The SLM 50 modulates the reflected signal beam with the arranged binary digital data for each page transferred from the data arranging block 110 to provide a modulated signal beam in the form of M×N binary pixel data for each page. The modulated signal beam is transferred to the storage medium 60 The reflection mirror 30 functions to change the reflection angle of the reflected reference beam by a small amount for data storage of different pages.
The interference pattern of the modulated signal beam interfering with the reference beam is stored in the storage medium 60.
When only the reference beam is irradiated onto the medium 60 in order to reconstruct the data written thereon, the reference beam is diffracted by the interference pattern stored in the storage medium 60 so that a check pattern corresponding to the M×N binary pixel data for each page may be restored. The check pattern is irradiated on the CCD 70 as restored data, which is then imaged by the CCD 70 and transferred to the data extracting block 120. In the data extracting block 120, the original data arranged at the data arranging block 110 is reconstructed from the restored data. The reference beam used for reproducing the data written in the storage medium 60 should be irradiated thereon at the same incident angle as that of the reference beam used in recording the data to be reproduced on the storage medium 60.
In the prior art data storage system described-above, the binary bits of the input data are recorded in the storage medium on the page-by-page basis. For example, assuming that each page to be stored is constituted by M×N binary pixels (i.e., M×N binary input data bits) as shown in FIG. 2, the binary input data bits to be stored are sequentially arranged at the data arranging block 110 in a zigzag scanning order along a column or a row direction in each page as shown in Table 1. That is, the first data bit is arranged at position (1,1) and the second data bit is arranged at (1,2) or (2,1) of the first page. If the second data bit is arranged at (1,2) for example, next data bits are arranged along the row direction, i.e., (1,3), (1,4), . . . , (1,N), (2,1), (2,2), . . . , (2,N), . . . , (M,1), (M,2), . . . , (M,N).
TABLE 1Data NumberPage NumberInput Coordinate11(1,1)21(2,1) or (1,2)31(3,1) or (1,3)41(4,1) or (1,4). . .. . .. . .MxN+12(1,1)MxN+22(2,1) or (1,2)MxN+32(3,1) or (1,3). . .. . .. . .(P−1)xMxN+1P(1,1)(P−1)xMxN+2P(2,1) or (1,2)(P−1)xMxN+3P(3,1) or (1,3). . .. . .. . .
The remaining input data are processed at the data arranging block 110 with respect to next pages in a manner described above. The data arranged based on the page basis is transferred to the SLM 50 and converted to the modulated signal beam in the form of binary pixels having “ON” or “OFF” images in accordance with the binary input data arranged at the data arranging block 110.
However, the conventional data arranging scheme described above suffers from certain drawbacks in that it is difficult to distinguish an “ON” state of pixel from an “OFF” state of pixel during the reproduction of the input data. Specifically, there normally is a large spatial variation across each page in the intensity of the modulated signal beam. That is, the light intensity is high at the central region thereof and decreases rather rapidly as proceeding toward the peripheral region thereof. Therefore, even a sequence of data consecutively written on a single column or row of a page can experience a large intensity variation in its “ON” pixel values (corresponding to binary “1” of the input data), rendering it difficult to identify an “ON” pixel and an “OFF” pixel in the reproduction process of the data.
For this reason, it would be desirable to provide a data input method for use in hologram digital data storage system capable of removing or reducing the adverse effect of a large spatial intensity variation of a light beam which complicates the reproduction process of the data in the system.